1. Field of the Invention
The present invention relates generally to a tracking servo system for an optical disk player appartus. More specifically, the invention relates to a tracking servo system which is particularly adapted for quick tracking by detecting warding pit in a servo area formed on an optical disk.
2. Description of the Prior Art
Known optical disks for high density recording of audio signals and/or video signals are designed to be formed with a plurality of data pits for recording audio and/or video data in digital fashion. In such optical disks, the data pits are formed by digging pits with an energy beam, such as an optical laser beam. Such optical disks are designed only for reproduction and not for recording. In order to make such optical disks more useful, an optical disk which allows both recording and reproduction of data has been developed. Such type of optical disk which allows both recording and reproduction of data will be hereafter referred to as a recordable optical disk.
The general idea of the recordable optical disk and the task of the invention will be discussed to facilitate a better understanding of the present invention, with reference to FIGS. 1 to 4, in which:
FIG. 1 is a general plan view of a recordable optical disk;
FIG. 2 is an enlarged illustration showing the structure of a recording track to be formed on the recordable optical disk;
FIG. 3 is a further enlarged illustration of pits in a servo area on the tracks of the recordable optical disk; and
FIG. 4 is a chart showing a waveform of a tracking error signal.
As shown in FIG. 1, the recordable optical disk is formed with a vertical magnetizing layer on the recording surface, which vertical magnetizing layer can be magnetized by an optomagnetic process. On this recordable optical disk, recording tracks are formed in a form shown in FIG. 2. In each of the recording tracks are servo areas for tracking with regular intervals. In each servo area, three depressed pits 2A, 2B and 3 are formed by an embossing process, for example. The depth of each depressed pit 2A, 2B and 3 substantially corresponds to .lambda.4 to .lambda.8 where .lambda.is a wavelength of a laser beam. As clearly shown in FIG. 3 the depressed pits 2A and 2B are respectively offset from the track center. The offset magnitude of the depressed pits 2A and 2B is respectively 1/4 of the distance Q between adjacent tracks. These depressed pits 2A and 2B will be hereafter referred to as servo pits. On the other hand, the depressed pit 3 is formed on the track center and is distanced from the servo pit 2B at a predetermined distance L. The signal reproduced from this depressed pit 3 serves as a reference clock of data. Therefore, this depressed pit 3 will be hereafter referred to as a "clock pit".
As seen from FIGS. 2 and 3, the servo pits 2A and 2B and the clock pits 3 on the respective recording tracks are oriented in radial alignment with each other. Namely, each of the servo pits 2A and 2B and the clock pit 3 are oriented at the same angular position to that of the adjacent tracks. The servo pits 2A and 2B and the clock pit 3 form one servo area.
Between adjacent servo areas, data recording areas 4 are formed on the track. For example, as shown in FIG. 2, the data recording area 4 is formed between the servo area where the servo pits 2A and 2B and the clock pit 3 are formed, and the servo area where the servo pits 2A' and 2B' and the clock pit 3' are formed.
Upon recording and reproduction of the data, a laser beam is irradiated onto the servo pits 2A and 2B and the clock pit 3 to reproduce tracking error signals and reference clock signals based on the reflected light beam. Furthermore, on the basis of the reflected light beam from a mirror portion L between the pits 2B and 3, a focus error signal is established.
The tracking error signal level is variable depending uon the position of the laser beam spot 5 relative to the track. The relationship between the tracking error signal and the laser beam spot position is shown in FIG. 3. As will be appreciated from FIG. 3, when the irradiation point of the laser beam spot moves in a direction shown by the arrow, the laser beam spot position tends to vary as shown in A, B and C of FIG. 3. Depending upon the laser beam spot position, the tracking error signal level tends to vary as shown in the curves a, b and c of FIG. 3, depending upon the relative position of the laser beam spot 5 and the servo pits 2A and 2B.
In the case of track A of FIG. 3, the laser beam spot 5 is precisely in an on-track position. Therefore, the tracking error signal level drops at substantially the same magnitude at each of the positions of the servo pits 2A and 2B because of substantially even reflected light intensity, as shown in curve a of FIG. 3. On the other hand, in the case of track B, since the laser beam spot 5 is offset from the track center toward the side where the servo pit 2A is formed, the signal level drop at the position of the servo pit 2A becomes much greater than that at the servo pit 2B due to difference of the reflected light intensity, which difference is caused by the difference between the irradiated pit areas, as shown in b of FIG. 3. Similarly, in the case of track C, since the laser beam spot 5 is offset from the track center toward the side where the servo pit 2B is formed, the signal level drop at the position of the servo pit 2B becomes much greater than that at the servo pit 2A, as shown in curve c of FIG. 3.
Therefore, by sampling the reflected light intensity Sa at the timing of T.sub.1 where the laser beam irradiates the servo pit 2A and the reflected light intensity Sb at the timing of T.sub.2 where the laser beam irradiates the servo pit 2B, and by deriving the difference between the reflected light intensities at T.sub.1 and T.sub.2, the offset magnitude of the laser beam spot relative to the track center can be obtained. The signal indicative of the difference between Sa and Sb serves as the tracking error signal. In the tracking servo system, the laser beam spot position is controlled so as to reduce the tracking error indicative value (Sa - Sb) toward zero so that the laser beam spot can be located at the on-track position. Such tracking servo systems have been disclosed in the U.S. Pat. Nos. 4,553,228 and 4,430,870.
On the other hand, as is well known, the tracking servo system of the optical disk player apparatus also peforms seek or track-access action for locating the laser beam spot on a desired one of a plurality of tracks by jumping tracks in a radial direction. Namely, as is well known, in the seek action, the laser beam spot position is shifted radially to be located on the desired track. This seek action takes place by driving a laser head in a radial direction by an acceleration pulse and a subsequent deceleration pulse. By the acceleration pulse, the laser head is driven in the radial direction toward the desired track. Subsequently, the deceleration pulse is generated to stop the laser head at the desired track. However, due to an inertia moment exerted on the laser head, it is difficult to stop the laser head at the desired position, at which the laser beam spot is located at the on-track position. Therefore, subsequent to the seek action, it becomes necessary to adjust the laser beam spot position to be located at the on-track position. This action will be hereafter referred to as "on-track adjustment". This necessarily prolongs seek time.
Further details of the seek action and subsequent on-track adjustment will be discussed herebelow.
Assuming the radial position of the laser beam spot 5 is v and the pitch of the tracks in the radial direction is Q, and further assuming that the laser beam spot 5 is located at the on-track position at the position v, the tracking error signal e.sub.t =Sa-Sb=0. On the other hand, when the laser beam spot 5 is offset from the track center, the value e.sub.t of the tracking error signal can be illustrated by a sine coefficient as follows: ##EQU1## According to the laser beam spot position, the tracking error signal value varies substantialy along the sine curve as shown in FIG. 4, in a stepwise fashion with a stepping magnitude of k.
During seek or traversing actions, when the laser beam spot is located at a position offset from the track center in a magnitude of Q/2, the laser beam spot irradiates the outer servo pit 2A of one track and the inner servo pit 2B of the next track. In this case, the irradiation area at the servo spot 2A and 2B becomes even and thus the tracking error indicative value becomes zero (0).
Therefore, in the operation in which the laser beam spot is shifted in a radial direction for track-jump action or so forth, the track error signal level in a range F of FIG. 4 becomes smaller toward zero as the laser beam spot approaches the track center. On the other hand, while the laser beam spot is in a range of N of FIG. 4, the tracking error signal e.sub.t increases as the laser beam spot approaches the track center. This necessarily prolongs the period necessary for on-track adjustment.
In order to solve this problem, the conventional optical disk player apparatus, such as that disclosed in the Japanese Patent Second (examined) Publication (Tokko) Showa 59-8893, uses an RF signal to be reproduced during reproduction for on-tracking adjustment. In the practical on-track adjustment action, the phase of the level variation of the RF signal which is caused when the laser beam spot offsets from the track center is compared with the phase of the tracking error signal for extracting the deceleration or braking signal.
This method is effective for shortening on-track adjustment time. However, such a process is possible when the transverse action and subsequent on-track adjustment action takes place in playback only on an optical disk, such as compact disks, video disks and so forth, since the RF signal can be reproduced. However, since the recordable optical disk has no RF data recording pit, this process is not applicable for the recordable optical disk.